Alkali metal borohydride-borane complexes



United States Pate ALKALI NIETAL BOROHYDRIDE-BORANE COMPLEXES Evelyn B.Baker, Richard B. Ellis, and William S. Wilcox,

Birmingham, Ala., assignors to Gallery Chemical Company, Pittsburgh,Pa., a corporation of Pennsylvania No Drawing. Application June 3, 1955Serial No. 513,141

Claims. (Cl. 260-6065) This invention relates to alkali metalborohydrideborane complexes and in particular it relates to a newcomposition of matter having the formula NaBH -BH 01' NQB2H7.

The chemistry of the alkali metal hydrides and borohydrides in generalhas developed rapidly in recent years. The first borohydride wasreported by Schlesinger in 1939. Methods for the preparation of all thesimple alkali metal hydrides and borohydrides having the generalformulas MH and MBl-l where M is an alkali metal are now known. Sincethe borohydride ion is extremely stable, the borohydrides are findingmore applications every day in chemical operations. Furthermore, thealkali metal borohydrides are much more soluble in common solvents thanare the corresponding hydrides. As a result, the borohydrides arespecific reducing agents for a Wide variety of organic compounds and arebeing used quite extensively for research in this field and to a lesserdegree for industrial applications. One particular use for which sodiumborohydride is admirably suited is as a starting material for thepreparation of diborane, B H and other boron containing compounds.However, none of the alkali metal borohydrides contain as high apercentage of boron as desired for preparative purposes. Therefore, itwould be highly beneficial if the bodohydrides as a class could becombined with the less stable borane group, EH to provide more boron permol as well as greater solubility in the solvents used.

It is an object of this invention to provide alkali metalborohydride-borane complexes wiich are more soluble in common solventsand contain more boron per mol than the brohydrides alone.

Anther object is to provide a method for preparing a solid complex ofsodium borohydride and diborane with a polyethylene glycol dialkyl ethersuitable for use as a means of transporting diborane in a solid formrather than as a gas.

A still further object is to provide a new and useful composition ofmatter having the empirical formula NHBZHPI.

Other objects will become apparent as the invention is disclosedthroughout the specification and appended claims.

This new composition of matter and method for preparing the same will bemore fully described hereinafter and the novelty thereof will beparticularly pointed out and distinctly claimed.

This invention is based upon the discovery that whenever a solublealkali metal borohydride, such as NaBI-L and diborane are broughttogether in solution in a polyethylene glycol dialkyl ether, a stablecomplex is formed in the proportion of two molecules of borohydride toone molecule of diborane. These proprtions are represented by theformula NaBi-l -Bl-i or NaB l-l This complex has been prepared insolution and in a solid crystalline form containing one molecule ofsolvent.

In one experiment, the existence of the complex NaBH -BH was confirmedby the electrolysis of a solution ofsodium borohydride i-n ethylene.glycol dimethyl ether. Diborane was produced at the'anode. About halfof the total quantity of electricity passed through the cell beforeappreciable amounts of diborane gas were evolved. Then diborane appearedin greated than Faradaic proportions. Thus, the borohydride ions weredischarged to form borane (EH molecules which combined immediately withthe remaining borohydride ions to form the complex BH -BH ions. Whenhalf of the borohydride had been discharged, the remaining BH; ions wereall in the complex form. .Subsequent discharge of the complex ionsproduced twice as much diborane as would come fromsimple BH; ions.

In another experiment, diborane was bubbled through a solution of 5.25millimoles of sodium borohydride in 10 millimoles of triethylene glycoldimethyl ether.for several hours. Only 2.6 millimoles of diborane wereabsorbed. This confirmed the formula for the complex formed as NaBHyBH-2CH (OC H OCH By careful evaporation of the solvent, one mol of theether can be removed leaving a white solid.

The complex NaBH -BH forms whenever diborane is bubbled through asolution of sodium borohydride in'a polyethylene glycol dialkyl ethersuch as the dimethyl ether of ethylene glycol, diethylene glycol,triethylene glycol and tetraethylene glycol. This complex also formswhen diborane or nascent borane is generated in such solutions of anyalkali metal borohydride by any of the known chemical or eletrochemicalreactions. The formation of this complex is much more rapid when nascentborane is presentsuch as in the reaction of an alkali metal borohydridewith a Lewis acid. The complex NaBH -BH has not been isolated as suchfree of ether but good Evidence for the existence of the NaBH -BHcomplex is based on several facts: (1) A solution of diethylene glycoldimethyl ether saturated with NaBH; alone at room temperature contains5% by weight of NaBH but when the same solution is also saturated withdiborane it contains 12% by weight of NaBH Thus, the complex formed isconsiderably more soluble than NaBH.,. (2) When a NaBI-I, solution indiethylene glycol dimethyl ether is saturated with diborane, theelectrical conductivity increases by a factor of 10 to 50 over a similarsolution saturated with NaBI-I, only. This increase is attributed to theformation of the complex. (3) Comparison of infra-red data on a sodiumborohydride-triethylene glycol dimethyl ether solution with a similarsolution containing the sodium borohydride-borane complex shows that theNaBH, remains essentially unchanged. However, a new band is observed inthe BH region of the complex which band is not congruous with BH ofNaBH, or B-H of diborane. B H is not present as such and no evidence forBOC bands can be found. This new band appears as a doublet in the BH;region which further substantiates the molecular formula for the complexas NaBH -BH (4) Chemical analysis of the complex always results in aboron to sodium ratio of 2:1 compared with a 1:1 ratio for NaBH This newcomposition of matter is useful in that it can be dissolved inpolyethylene glycol dialkyl ethers to produce a reducing solution havinga hydridic hydrogen content of more than twice that of a solution ofNaBH; in the same ether. When isolated as a solid etherate, the sodiumborohydride-borane complex can be used to transport diborane in aconvenient manner. Such etherates when heated to 80 C. will regeneratediborane leaving a solution of NaBH in the ether used to make theoriginal complex.

While this invention has been described using sodium borohydride as atypical alkali metal borohydride soluble in the polyethylene glycoldialkyl ethers, other experiments with solutions of other soluble alkalimetal borohydrides show that the borane complexes of potassium andlithium borohydrides may be prepared in a similar 'manner. It shouldtherefore be understood that within 'the scope of the appended claims,this invention may be practiced otherwise than as specificallydescribed.

What is desired to be claimed and secured by United States patent is:

1. A method of preparing a stable 'alkali metal borohydride-boranecomplex which comprises reacting diborane with an alkali metalbo'rohydride in a polyethylene glycol dialkyl ether having the formulawhere R is a lower alkyl radical and n is a number from 1 to 4 to formin situ a complex having the general "formula MB H where M is an alkalimetal.

- 2. A methodaccording to claim 1 in which the alkali metal borohydrideis selected from the class consisting of sodium, potassium and lithiumborohydrides.

3. A method according to claim 1 in which the ether used is apolyethylene glycol dimethyl ether having the formula CH (OC H OCH wheren is an integer from 1 to 4. 4. A method of preparing a solidcrystalline sodium -borohydride-borane-etherate complex which comprisesbubbling diborane through a solution of sodium borohydride in apolyethylene glycol dimethyl ether having the formula CH (OC H ),,OCHwhere n is an integer from 1 to 4, then passing diborane rapidly overthe complex formed to evaporate the excess solvent and recovering thecomplex as a solid residue.

5. A method according to claim 4 in which the ether used is ethyleneglycol dimethyl ether and the solid complex obtained has the formula 6.A method according to claim 4 in which the ether used is triethyleneglycol dimethyl ether and the solid complex obtained has the formula 7.A solution consisting essentially 'of the borohydrideborane complexhaving the empirical formula NaBzHq in a polyethylene glycol dialkylether having the formula RO (OC H ),,R, where R is a lower alkyl radicaland n is a number from 1 to 4.

' 8. The compound having the molecular formula NaBH -BH solvated with apolyethylene glycol dialkyl ether having the formula RO(OC H R, Where Ris a lower alkyl radical and n is a number from 1 to 4.

9. The compound having the molecular formula NaBHyBH -xCH (OC H OCHwhere x is a number from 1 to 2.

10. The compound having the molecular formula NaBH 'BH -xCI-I (OC H )OCHwhere x is a number from 1 to 2.

References Cited in the file of this patent UNITED STATES PATENTS2,545,633 Schlesinger et a1. Mar. 20, 1951 OTHER REFERENCES Bulletin502A on Sodium Borohydride, pub. by Metal Hydrides, Inc., 12-24 CongressSt., Beverly, Mass., and consisting of two (2) pages. Received in U.S.Patent Ofiice, Div. 59, Feb. 15, 1950.

7. A SOLUTION CONSISTING ESSENTIALLY OF THE BOROHYDRIDEBORANE COMPLEXHAVING THE EMPIRICAL FORMULA NAB2H7 IN A PLOYETHYLENE GLYCOL DIALKYLETHER HAVING THE FORMULA RO(OC2H4)NR, WHERE R IS A LOWER ALKYL RADICALAND N IS A NUMBER FROM 1 TO 4.